As far as getting a position is concerned: Ensure that your GPS is set to transmit NMEA out, and that it is transmitting the required NMEA sentences. This will be in a place like the "Communications" menu depending on the brand of GPS.
Once you have done that a 'crude" check is to connect a multimeter to the NMEA out wire and the common ground from the GPS and check to see whether you get a reversing polarity signal on that line. Depending on the GPS it will be in the range of +5 to -5 and rapidly reversing. It's easiest to see with an analogue meter but a digital will do.
As for the MMSI number ICOM will have to tell you that. In the ICOM M502 unit it is in the setup menu from memory. To enter this menu on the M502 you turn the unit off and while holding the DSC/ENT button you turn to power on. Don't know whether that is relevant in this case.
Page 3 of this manual covers entry of an MMSI for the DS100
Best of luck.